1. Field of the Invention
This invention relates to a binder composition and an aqueous coating composition comprising the binder composition as a binder component. More particularly, it relates to a binder composition which is added to an aqueous coating composition to impart water resistance, heat resistance and weather resistance and is stable even in an alkaline region.
2. Prior Art
In conjunction with the recent increasing public concern about environment protection, active research works have been made to substitute aqueous systems for solvent system paint and coating compositions. Aqueous systems have a number of benefits including safe operation and the elimination of the hazards of air pollution, explosion and fire evoked by the volatilization of solvents.
Additives are often added to such compositions for imparting water resistance, heat resistance and weather-ability thereto. In order that additives be used in aqueous systems wherein water accounts for the majority of diluent, the additives must be soluble in water. Typical additives are silicone compounds and when used in solvent systems, they are effective for imparting water resistance, heat resistance and weatherability. However, few silicone compounds are stable in water and if any, they are just slightly stable under weakly acidic conditions and lose stability if the system turns alkaline.
As a solution to such problems, JP-A 279678/1994 proposes an aqueous surface treating composition comprising a major component obtained by reacting an aminotrialkoxy-silane with an organic compound capable of reacting with the amino group, hydrolyzing the reaction product, and optionally further reacting the hydrolyzed product with an organometallic compound such as a tetraalkoxysilane. The major component, however, is less water soluble since the amino group which largely governs water solubility has been reacted with an organic group. To compensate for the loss of water solubility, an anionic surfactant is added to the treating composition as a stabilizer. When the composition is used as a binder, the inclusion of an anionic surfactant can exacerbate some properties such as water resistance and heat resistance. Additionally, the treating composition is not so stable over a long period of storage and is unstable in an alkaline region. For these reasons, the composition is not adequate as a binder.
On outdoor applications, silicone coatings on substrates do not prevent the substrate surface from being discolored or degraded in a substantial sense since silicone compounds as such cannot block or absorb sunlight.
U.S. Pat. No. 4,395,461 proposes to impart improved weatherability to a polycarbonate substrate by (1) treating a polycarbonate substrate with a solution of the hydrolysis product of a silylated organic UV screen and curing the applied hydrolyzate on the treated substrate and (2) applying a silicone hardcoat composition onto the treated polycarbonate substrate and thereafter heating the resulting composite until the applied hardcoat composition is cured. This method, however, uses an organic solvent and requires two layer coating. JP-A 291667/1996 discloses to impart weatherability and durability to a substrate by treating a substrate surface with a silicone primer coating solution containing a UV absorber and further forming a topcoat of a silicone hardcoat composition. This method also uses an organic solvent and requires two layer coating. Further, JP-A 279678/1994 discloses an alcohol/water base UV-resistant coating composition obtained by the hydrolysis of an alkoxysilane and a hydroxy group-containing organic UV absorber. The organic UV absorber in this composition is bonded through a SiOC linkage which is susceptible to hydrolysis and lacks durability.